This invention concerns a method of producing surgical grade calcium sulphate and particularly but not exclusively a method of producing a resorbable osteoconductive bone void filler material.
Plaster of paris (hemi-hydrate calcium sulphate) is known to be suitable for use as a filler for bone defects. This material provides a number of required characteristics and does not produce any significant undesirable reaction within a body. Traditionally plaster of paris is produced from naturally occurring gypsum. This often has associated undesirable impurities. For medical applications and particularly for implantation high purity materials are obviously needed. Whilst it is possible for calcium sulphate to be produced synthetically, conventional techniques produce a material which has a high surface area which thus has a high water demand and therefore produces a low strength material when set. Such set material has a high dissolution rate and too short a residence time when implanted.
Plaster of paris, or more accurately calcium sulphate, has two modes of use as a bone filler. The first is in the di-hydrate form, usually as pellets which can be placed in a bone void. The second is as a hemi-hydrate powder which can be made up with water into a paste, which paste is mouldable to a required shape. Such materials are usable in the fields of orthopaedic, dental and maxillofacial surgery. Hemi-hydrate calcium sulphate has two forms. The first xcex1 is produced by hydrothermal treatment of the di-hydrate. The second xcex2 is produced by dry heat calcination. This latter form tends to have a low density, a higher water demand and therefore a lower strength and a faster dissolution rate than the xcex1 form.
According to the present invention there is provided a method of producing surgical grade calcium sulphate, the method comprising forming an initial calcium sulphate di-hydrate from synthetic constituents; dehydrating the initial calcium sulphate di-hydrate to form calcium sulphate anhydrite; and subsequently rehydrating the calcium sulphate anhydrite and allowing subsequent calcium sulphate di-hydrate to crystallise out.
The subsequent di-hydrate may be used as a solid material bone filler and may be formed into pellets. The crystallised subsequent calcium sulphate di-hydrate may be ground prior to forming into pellets.
Alternatively, the subsequent calcium sulphate di-hydrate may be calcined to form calcium sulphate hemi-hydrate, which material can be mixed with water or a salt solution to form a settable paste.
The calcining may be hydrothermal to form a calcium sulphate hemi-hydrate, and may be carried out in an autoclave. The calcining may be carried out at a pressure of 1-6 bar, and desirably 2-3 bar. The calcining is preferably carried out for a half to five hours, and desirably one to two hours.
Alternatively, the calcining may be carried out in dry heat conditions. The calcining may be carried out at a temperature of 70-200xc2x0 C. and desirably at 150-175xc2x0 C., and for a period of a half to six hours and desirably one to two hours.
Following calcining, the calcium sulphate hemi-hydrate is preferably ground to a powder, and desirably with a particle size of less than 150 microns.
The initial calcium sulphate di-hydrate may be formed by mixing soluble calcium and sulphate salts such that calcium sulphate precipitates out. The di-hydrate thus formed may be washed, and subsequently filtered, crushed and/or dried.
The calcium salt may be a chloride or nitrate. The sulphate may be a sodium, potassium or ammonium salt. The calcium and sulphate salts are preferably provided in a substantially equal molecular ratio.
Alternatively the initial calcium sulphate di-hydrate may be formed from neutralising lime with sulphuric acid.
The dehydration of the initial calcium sulphate di-hydrate preferably takes place within a temperature range 110-350xc2x0 and desirably at less than 300xc2x0 C., to form soluble calcium sulphate anhydrite. Alternatively the dehydration may take place at a temperature above 350xc2x0 C. to form insoluble anhydrite.
The dehydration of the initial calcium sulphate di-hydrate by the application of heat may take place in an open container, or in a closed container, or hydrothermally in the presence of steam.
The rehydration of the calcium sulphate anhydrite preferably takes place immediately following dehydration. The calcium sulphate anhydrite is preferably fully immersed in water or an aqueous solution for rehydration.
The rehydration may take place in water or a dilute salt solution. The salt solution may comprise succinic acid or a potassium sulphate solution, with a concentration of less than 1% and desirably substantially 0.1%.
Finely powdered calcium sulphate di-hydrate may be added to be present during rehydration such that the powdered calcium sulphate acts as crystal seeds, and the addition may be in the order of 5 g per liter of water. For soluble anhydrite the rehydration preferably takes less than five days, and for insoluble anhydrite preferably more than five days.
The subsequent calcium sulphate di-hydrate is preferably dried following crystallisation. Alternatively, if it is to be autoclaved to form xcex1 hemi-hydrate it may be held in a damp condition.